Actin has been identified in boar and bull spermatozoa by indirect immunofluorescent labeling with deoxyribonuclease (DNAse), rabbit anti-DNAse and fluorescein-labeled anti-rabbit immunoglobin. Fluorescent label is an indication of actin by these criteria: 1) tight specific affinity of DNAse for actin; 2) demonstration of labeled filament bundles in fibroblasts; 3) decrease in label by competition with muscle actin. The actin is located in the post-acrosomal region of bull and boar sperm heads in testicular, epididymal and ejaculated spermatozoa. It is not filamentous nor significantly redistributed during maturation. Actin is also located in the cytoplasmic droplet of epididymal boar and bull spermatozoa. It also is not filamentous. Actin is present in fragments of Sertoli cells associated with spermatids dispersed from the testis. A myosin-like, actin-stimulated adenosine triphosphate hydrolase (ATPase) activity can be demonstrated in bull sperm extracts.